Various methods and systems have been adopted in the vibration testing industry to simulate vibration and shock environments. There is a wide range of potential uses for such methods and systems, from earthquake simulators to the development of vibration-tolerant product designs. For example, a number of industries use vibration testing for determining their products' effectiveness and longevity when subjected to these environmental extremes. Vibration testing may be conducted to verify that a specific design will survive in its intended vibration environment, or to screen out defective parts at an early stage in the manufacturing process.
Vibration testing has traditionally been done with the test article restrained to move in a single axis. However, it has been shown that vibration testing in three mutually exclusive axes simultaneously can simulate real world conditions better than single axis testing. Therefore, to better simulate real world conditions, it is often desirable to use multiple-axis, multiple degree-of-freedom vibration test fixtures.
One such multiple-axis, multiple-degree-of-freedom vibration test fixture is disclosed in U.S. Pat. No. 5,343,752 to Woyski et al., and illustrated herein as FIGS. 1 and 2. The fixture 10 includes a normally horizontal upper fixture base 12, supported by three sets of upwardly extending pairs of rigid support arms 14, 16, 18. The support arms are mounted to a rigid stationary lower base 20 so as to accommodate vibration motion of the upper base in both the vertical plane and the horizontal plane. A hydraulic vibration actuator module 22 is affixed to each support arm, each module having a double-acting piston actuator assembly 24 reciprocating along a horizontal axis and a double-acting piston actuator assembly 26 reciprocating along a vertical axis. This permits the vibration table to be vibrated along orthogonal x and y-axes and rotated with a z-axis rotation in the horizontal plane by corresponding combinations of vibration of the three horizontal piston actuator assemblies. Separately, components of vertical forces are induced by the three sets of vertical piston actuator assemblies to oscillate the table in various combinations of pitch, yaw or heave orientations in the vertical plane, combined with horizontal plane movement.
Each of the hydraulic piston actuator assemblies has a split-piston design, wherein a pair of pistons are aligned along a common axis of vibration, and opposite end portions of the pistons project outwardly from opposite sides of the actuator housing. The pistons each reciprocate in a corresponding cylinder contained internally within the actuator housing as hydraulic fluid acts on ends of the pistons.
Although the split-piston actuator design has certain advantages, it has been found that there are some disadvantages associated with this design, especially in longer stroke applications (i.e. greater than 4 inches). Longer strokes often create higher side or bending loads on the actuator when used in multi-degree of freedom machines. Various features of the split-piston actuator design result in relatively poor side load capability in the longer stroke applications. For example, in order to accommodate such longer strokes, the overall length of the actuator must be increased, and the side loads on the actuator become extremely difficult to handle. Additionally, the split-piston actuator design does not allow the piston rod diameter to be changed in order to improve the side load capability of the actuator. Since the hydraulic fluid acts on one end of the piston rod, the diameter of the piston rod is determined by the required actuator force. Moreover, the bearings supporting each of the pistons in the split-piston actuator design are relatively close together, thus intensifying the existing problems associated with the side load capability of the actuator.
Consequently, a need exists for an improved hydraulic vibration actuator capable of addressing the disadvantages associated with the prior art actuator designs identified above. More specifically, a need exists for a hydraulic vibration actuator having increased side load capability for use in long stroke vibration testing applications.